Production and characterization of maize chromosome 9 radiation hybrids derived from an oat-maize addition line

In maize (Zea mays L., 2n = 2x = 20), map-based cloning and genome organiza tion studies are often complicated because of the complexity of the genome. Maize chromosome addition lines of hexaploid cultivated oat (Avena sativa L., 2n = 6x = 42),where maize chromosomes can be individually manipulated, represent unique materials for maize genome analysis. Maize chromosome addi tion lines are particularly suitable for the dissection of a single maize c hromosome using radiation because cultivated oat is an allohexaploid in whi ch multiple copies of the oat basic genome provide buffering to chromosomal aberrations and other mutations. Irradiation (gamma rays at 30, 40, and 50 krad) of a monosomic maize chromosome 9 addition line produced maize chrom osome 9 radiation hybrids (M9RHs)-oat lines possessing different fragments of maize chromosome 9 including intergenomic translocations and modified ma ize addition chromosomes with internal and terminal deletions. M9RHs with 1 to 10 radiation-induced breaks per chromosome were identified. We estimate d that a panel of 100 informative M9RHs (with an average of 3 breaks per ch romosome) would allow mapping at the 0.5- to 1.0-Mb level of resolution. Be cause mapping with maize chromosome addition lines and radiation hybrid der ivatives involves assays for the presence or absence of a given marker, mon omorphic markers can be quickly and efficiently mapped to a chromosome regi on. Radiation hybrid derivatives also represent sources of region-specific DNA for cloning of genes or DNA markers.